Grokking Algorithms

Chapter 10
 
 
I
 
 
k-nearest neighbors
Picking good features
To figure out recommendations, you had users rate 
categories of movies. What if you had them rate pictures 
of cats instead? Then you’d find users who rated those 
pictures similarly. This would probably be a worse recommendations 
engine, because the “features” don’t have a lot to do with taste in 
movies!
Or suppose you ask users to rate movies so you can give them 
recommendations—but you only ask them to rate 
Toy Story
, 
Toy Story
2
, and 
Toy Story 3
. This won’t tell you a lot about the users’ movie tastes!
When you’re working with KNN, it’s really important to pick the right 
features to compare against. Picking the right features means
• Features that directly correlate to the movies you’re trying to 
recommend
• Features that don’t have a bias (for example, if you ask the users to 
only rate comedy movies, that doesn’t tell you whether they like 
action movies)
Do you think ratings are a good way to recommend movies? Maybe I 
rated 
The Wire
more highly than 
House Hunters
, but I actually spend 
more time watching 
House Hunters
. How would you improve this 
Netflix recommendations system?
Going back to the bakery: can you think of two good and two bad 
features you could have picked for the bakery? Maybe you need to make 
more loaves after you advertise in the paper. Or maybe you need to 
make more loaves on Mondays. 
There’s no one right answer when it comes to picking good features. You 
have to think about all the different things you need to consider.
EXERCISE
10.3
Netflix has millions of users. The earlier example looked at the five 
closest neighbors for building the recommendations system. Is this 
too low? Too high? 

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Introduction to machine learning